1. Field of the Invention
This disclosure relates generally to methods for determining the lithology and mineralogy of a subterranean formation, and more particularly, to methods for determining the general lithology, specific lithology and mineralogy of a subterranean formation using artificial intelligence systems and well log data.
2. Description of the Related Art
There has long been a need for open-hole logging tools and methods that would be capable of providing measurements of the lithology and mineralogy of a geologic formation in selected directions, providing measurements of the both the mineralogy and lithology both close to the bore hole and deep into the subterranean formation, and provide all such measurements with high vertical and lateral resolution. Quantitative information about the reservoir rock lithology and associated minerals is important not only for determining the producing potential for a specific formation, but for making technical and business decisions in hydrocarbon exploration and exploitation as well. For example, exploration geologists can use rock mineralogy information associated with subterranean formations to reduce the risk in discovering hydrocarbons by determining the thermal and diagenetic history of the specific formation, defining the provenance (source area) and the depositional environments of the sediments in the formation, and correlating certain minerals with well logs. Formation mineralogy information can also be used during the exploration process to assess reservoir quality, develop effective depletion strategies, and predict the effect of rock-fluid interactions, while during the production process it can be used to design work-over and completion strategies, such as selection of drilling fluids and proper stimulation methods (e.g., effective acidizing or fracturing applications).
The interpretation of formation lithology, both general and specific, is also important. For example, quantitative knowledge of the lithological constituents present in a subterranean formation surrounding a well, as a function of depth, could be valuable in assessing all aspects of exploration, evaluation, production and completion. For example, suitable applications could include regional studies of facies architectures, estimating distributions of reservoir facies, establishing quantities of clay materials in all layers, identifying subtle and pronounced changes in depositional or diagenetic facies by characterizing the formation minerals, and planning enhanced recovery strategies.
Traditional methods of determining subterranean formation lithology and mineralogy have used cores from wellbores, which are often analyzed using X-ray diffraction techniques and the like. However, such traditional methods are very time-consuming, and are not efficient for use in exploration applications. Consequently, through the use of a variety of logging tools, numerous attempts to estimate, evaluate, and interpret both the lithologies and the mineralogy of subsurface formations by transforming log data into lithology and/or mineralogy logs have been made.
For example, methods have been suggested for the in situ examination of subsurface formations penetrated by a borehole in order to ascertain the cation-exchange capacity of such formations within select geological regions. Using natural gamma ray logging, signals were developed that functionally relate to the total gamma radiation and to the potassium, uranium and thorium energy-band radiations. According to these methods, the cation-exchange capacities of core samples can be determined by correlation with selected parameters provided by the gamma ray spectrometer to establish functional relationships. Cation-exchange capacities of formations in subsequent boreholes within the same and surrounding regions can then be determined in situ by use of the natural gamma ray spectrometer and these established relationships. This technique is of seemingly limited utility, however, because cation-exchange capacity is being reportedly correlated to elements that generally have very little global relation to clay or other similar minerals that dictate cation-exchange capacity.
Other methods described in the art provide for quantifying and characterizing mineral content of a subterranean formation as a function of well depth. According to these methods, elemental data derived from logging tools can be input into an element-mineral transform mathematical operation, such as a matrix of the type constructed using multivariate statistical analysis methods, in order to determine the quantity of at least one or more of the dominant minerals within in the formation under evaluation. From both the mineral quantity information and the elemental log data, the formation minerals can be predicted or hypothesized. Other related methods and associated apparatus suggest methods and apparatus for determining formation lithology using gamma ray spectroscopy, using inelastic scattering gamma ray spectra taken in a borehole and analyzed by a least squares spectral fitting process to determine the relative elemental contributions thereto of chemical elements postulated to be present in unknown earth formations and contributing to the measured spectra from the formations. In some reports, based on the calibrated inelastic yields for selected elements, calibrated estimates of the elemental yields from measured thermal neutron capture gamma ray spectra may also be determined, from which further information concerning formation lithology may be derived or theorized.
More recently, several methods for quantifying the lithologic composition of formations surrounding boreholes have been suggested. Such methods typically involve the construction of two or more lithology compositional models from known well log data for a formation, and the subsequent combination of the models in order to determine a range of possible solutions having an upper limit defined by a pure component model and a lower limit defined by a proportional mixture model, thus allowing the maximum concentration of any lithologic component to vary between 0% and 100%.
Other reports directed to the estimation of mineralogy have been reported by Harvey, et al. [SPWLA 33rd Annual Logging Symposium, pp. 1-18 (1992); and Core-Log Integration, Geological Society (London), Vol. 136: pp. 25-38 (1998)], as well as by Hertzog, et al. [Society of Petroleum Engineers. SPE paper No. 16792, pp. 447-460 (1987); SPE Formation Evaluation, Vol. 4, pp. 153-162 (1989)]. Several of these techniques describe the use of pulsed neutron devices, direct activation of the formation, and the natural gamma spectra of the formation, for use in obtaining continuous well logs of the major element chemistry of a formation. These tools and methods offer measurements of Si, Al, Ti, Fe, Ca, K, S and the minor elements Gd, Th and U, together with H and Cl. Transformation of the major elements into the more conventional oxide forms provides virtually complete major element oxide analysis at each measured depth interval down the borehole. However, the transformation of a rock's elemental composition to mineral and lithological assemblages has been the subject of numerous approaches, ranging from linear programming and genetic algorithms to numerical models such as least squares minimization.
For example, element to mineral transformation algorithms, used for quantifying minerals from downhole nuclear spectroscopy elemental data, have had limited success in representing the bulk chemical composition of a rock in terms of its mineralogy. More specifically, because the minerals of rock matrices contain many of the same elements in their crystal structures, quantification-type methods for determining minerals in subterranean rock formations, e.g., silicate minerals, using only chemistry or chemistry-based methodologies, without a priori knowledge of the minerals present, can result in problems involving non-unique solutions resulting from compositional colinearity [see, Harvey, P. K., et al., Developments in Physics, Vol. 122: pp. 141-157 (1997); and, Lofts, J. C., et al., Nuclear Physics, Vol. 8: pp. 135-148 (1994)]. This challenge can in turn result in a poor estimate of those phases having similar compositions, which then in turn leads to errors in quantifying other phases in the rock, a problem which magnifies exponentially for each quantification process. In particular, element-to-mineral transformations using traditional, least squares methods and the like have been found susceptible to colinearity, rendering them substantially unreliable for mineral quantification [Chakrabarty, T., et al., J. Can. Petroleum Technology, Vol. 36: pp. 15-21 (1997)].
Further, many of the existing logging tools and methods, such as those described briefly herein, are unable to provide the adequate penetration into the geologic formation surrounding the borehole necessary to provide the requisite detailed geological information many well-log operators and analysts are looking for. In addition, many existing logging tools are not directional, and the resolution of measurements is also limited, particularly at greater distances into the geologic formation. Further, and perhaps more importantly, existing methods for determining subterranean lithology and/or mineralogy are based on determining the mineralogy of the formation first, and then attempting to determine or correlate the lithology to the mineralogy. However, this is severely limiting, as errors in determining the mineralogy (such as errors that can occur in transforming the major elements into the more conventional oxide forms) can translate into significantly erroneous lithology characterizations.
This application for patent discloses methods for the determination of subterranean formation mineralogy from formation lithology data, using an artificial intelligence system which uses elemental measurements obtained from downhole tools comprising pulsed neutron devices to generate algorithms which can then be used to define the general lithology, then the specific lithology, and finally the mineralogy of a subterranean formation surrounding a wellbore or similar earth borehole.